Changes in engine speed due to frequent, quick application or release of an accelerator leads to excess energy consumption in a hybrid electric vehicle having an internal combustion engine and a motor.
With a hybrid electric vehicle disclosed in JP 2001-164960 A, published by the Japan Patent Office in 2001, wasteful energy consumption is suppressed by delaying the response speed for the engine speed to prevent frequent changes in transient operating states where the required drive force increases. The drive force by the motor is increased instead of increasing an engine output power providing that the state of charge of battery, namely the electric power capable of being output by the battery, is large. By supplementing the drive force by the motor, the response speed for the engine speed can be delayed. However, when the state of charge of battery is small, it is necessary to quicken the response of the engine speed, because the motor cannot supplement the drive force. Accordingly, the response speed for the engine speed is delayed according to the state of charge of battery.